If you have hearing or speech difficulties, or want to call someone who does, you can use Real-Time Text (RTT) for your phone calls if your carrier supports it.

Note: To make or receive an RTT call, you must be using macOS Mojave 10.14.2 or later with Wi-Fi Calling through iPhone with iOS 12.1.1 or later. This feature is not available in all countries or regions or supported by all network providers. Depending on your network environment, an RTT call may be downgraded to a textphone call.